1. Field of the Invention
The invention relates to a method of determining the throughflow (flow rate) of a gas mixture as well as uses of this method, in particular at a petrol station.
2. Description of Related Art
When filling up a motor vehicle at a petrol station, fuel is poured into the motor vehicle""s tank from a petrol pump with the help of a discharge valve. At the same time, the gas mixture which is above the liquid level of the fuel in the motor vehicle""s tank and consists of a fuel vapour/air mixture is sucked out via a separate line and conducted into the storage tank of the petrol station. The gas pump used for this should be controlled such that the volume of fuel vapour/air mixture sucked out per time unit is equal to the volume of fuel poured into the tank of the motor vehicle per time unit.
A method is known from WO 98/31628 for controlling the gas pump in relation to the delivery rate of the fuel pump used to supply the fuel. The throughflow of the sucked-out fuel vapour/air mixture is measured using a so-called Fleisch tube as a sensor, in order to volumetrically match the delivery rate of the gas pump to that of the fuel pump. A fundamental problem with sensors for measuring the throughflow of a gas is that the measurement signal emitted by the sensor can depend on the composition of the gas, but this composition is often not precisely known, so that the measurement signal cannot be corrected accordingly.
It is the object of the invention to create a method of determining the throughflow of a gas mixture which can be carried out economically and reliably using a relatively simply designed flow sensor even when the composition of the gas mixture is not known or fluctuates.
The object is achieved by a method including the features of determining the throughflow of a gas mixture, and a device for carrying out the method. An application of the method may be implemented in many ways in particular at a petrol station. Advantageous versions of the invention result from further embodiments.
With the method according to the invention of determining the throughflow of a gas mixture, the gas mixture flows past a flow sensor. The flow sensor has a heating device and a first temperature probe reacting to the temperature of the heating device. A second temperature probe measures a temperature which is characteristic of the temperature of a liquid above which the gas mixture stands as vapour. A first measurement signal characterizing the composition of the gas mixture as well as its heat dissipation capacity is produced by means of the known vapour-pressure curve of the liquid and the temperature of the liquid. A second measurement signal characterizing the throughflow of the gas mixture is produced by means of the heating power supplied to the heating device, the temperature of the first temperature probe and the first measurement signal.
The flow sensor used in the method is a conventional thermal flow sensor. To measure the throughflow, the cooling effect exerted by the gas mixture flowing in the flow sensor is exploited. The greater the flow rate and thus the throughflow of the gas mixture, the greater the amount of heat removed per unit of time from the flow sensor by heat transmission and convection via the gas mixture. Thus if e.g. a constant heating power is supplied to the heating device, the temperature recorded by the first temperature probe is lower with a high throughflow than with a low throughflow. If, on the other hand, as is the case with a preferred version, the heating power of the heating device is controlled such that the temperature of the first temperature probe lies above the ambient temperature by a predetermined value (therefore it is essentially constant), a higher heating power is accordingly necessary for a higher throughflow than for a lower throughflow.
However, the mentioned cooling effect or the heat dissipation capacity of the gas mixture depends on the composition of the gas mixture. If the vapour-pressure curve of the liquid above which the gas mixture stands as vapour is known, the vapour pressure of the liquid can be estimated by means of the temperature measured with the help of the second temperature probe, which in turn is characteristic of the composition of the gas mixture and thus its heat dissipation capacity.
The method can be used for example when the gas mixture is a fuel vapour/air mixture which develops above a liquid fuel. The higher the temperature, the greater the partial pressure and thus the proportion of the fuel in the fuel vapour/air mixture.
The first measurement signal and the second measurement signal are preferably produced using a control and evaluation apparatus which can include a microprocessor. The control and evaluation apparatus permits a preferably completely automatic operation of the method. In a preferred version, the throughflow of the gas mixture is determined from the second measurement signal and parameters determined during calibration measurements. With the help of such calibration measurements, using predetermined liquids under predetermined conditions, the parameters involved in the evaluation of the measurement results can be established, so that it is possible in principle to output the throughflow of the flowing gas mixture directly from the control and evaluation apparatus, e.g. onto a display or into a memory. However, a relative measurement for the throughflow is often already sufficient, e.g. if the method is used for control or regulation purposes.
Due to the thermal inertia of the components of the flow sensor as well as the second temperature probe, it can take some time, e.g. a few seconds, until the second measurement signal reliably characterizes the throughflow of the gas mixture. When the method is used for regulation purposes, this behaviour is disadvantageous, in particular if the process to be regulated is itself of only a relatively short duration. With an advantageous version of the method according to the invention, on the other hand, a much shorter reaction time can be achieved. After the insertion of the flow of the gas mixture, a forecast value for the second measurement signal and/or the throughflow of the gas mixture is determined, from the time-related pattern of the second measurement signal, which characterizes a stationary state. The forecast value can be determined e.g. from the first time-related derivative of the second measurement signal and/or the throughflow of the gas mixture. The evaluation steps necessary for this, in which the results of the calibration measurements can be also included, can be carried out e.g. with the above-mentioned control and evaluation apparatus.
The heating device of the flow sensor preferably has a PTC resistor as heating resistor. A PTC resistor is normally taken to mean a resistance component (as a rule made from ferroelectric ceramic, in particular doped barium titanate) in which the electric resistance rises steeply with the temperature from a certain temperature. Such a heating resistor is particularly suitable if the gas mixture is an explosive gas, as is mostly the case with a fuel vapour/air mixture. Because of its characteristic curve, a PTC resistor can actually be operated in its working range at a relatively high temperature, which must however lie below the ignition temperature of the gas mixture. If the temperature of the heating resistor were to increase because of an error in the control or regulation of the heating power supplied to the heating resistor, the resulting resistance value would rise steeply. Due to the upper voltage limit predetermined by the voltage supply, this leads to a decrease in the heating current, so that the highest temperature of a PTC resistor is limited and, with suitable dimensioning and choice of working range, safely lies below the ignition temperature of the gas mixture. The system is thus stable per se without an additional safety device and thus very reliable and safe.
A preferred use of the method according to the invention occurs in a method for monitoring the return of the gas in a filling-up system. The filling-up system is set up to convey a liquid fuel by means of a fuel pump from a storage tank into a tank to be filled up and to conduct the gas mixture standing above the fuel in the tank to be filled into the storage tank, by means of a gas pump. The throughflow of the gas mixture on the way into the storage tank is determined using the method according to the invention. For example, the filling system is allocated to a petrol station, the known vapour-pressure curve is the vapour-pressure curve of summer fuel or winter fuel depending on the time of year and the gas mixture is a fuel vapour/air mixture. In this example, the tank to be filled is the tank of a motor vehicle.
If the throughflow of a fuel vapour/air mixture as well as its composition is known, it can be established what amount of hydrocarbons is returned into the storage tank, which is of interest to the operator of the filling system. Moreover, the throughflow of the gas mixture measured using the method according to the invention can be used in an advantageous way to regulate the gas pump (see below).
The second temperature probe should be arranged near to the tank to be filled so that it records the temperature of the fuel as accurately as possible. It is however acceptable if it is located outside the tank. This simplifies the structure of the filling system, in particular if the tanks to be filled are of different designs.
A normal control of the gas pump in accordance with the throughflow of the fuel measured by a fuel flowmeter does not, in most cases, lead to the generally desired result, namely that the volumetric throughflow rate of the fuel supplied to the tank is equal to that of the gas mixture sucked out by the gas pump. Therefore the delivery rate of the gas pump is preferably controlled to match the delivery rate of the fuel via the throughflow of the gas mixture measured by the method according to the invention. The delivery rate of the fuel pump thus serves as a target value, the measured throughflow of the gas mixture is the actual value, and the drive of the gas pump is triggered according to the deviation of actual value from the target value according to one of the usual regulation processes.
If the delivery rate of the gas pump is not controlled, or to supplement a regulation, it is also conceivable that the control of the gas pump is intermittently adjusted to match the delivery rate of the fuel pump via the throughflow of the gas mixture determined by the method according to the invention. The control of the gas pump can be adjusted, e.g., after each filling procedure according to the throughflow of the gas mixture measured in stationary state during this filling procedure. This assumes that a short-term regulation of the gas pump is not necessary during a filling procedure, but that nevertheless the properties of the gas pump change over time. This is taken into account by adjusting the control of the gas pump according to the last throughflow measurement, e.g., by providing the control means, after each filling procedure (or more generally, at regular intervals) via an interface with new reference data, say for the drive current of the gas pump according to the delivery rate of the fuel pump or drive current of the fuel pump. If the gas pump is not directly regulated, the above mentioned disadvantage that the method of measuring the throughflow can have a time constant of some seconds because of the thermal inertia of components, does not play a part.
It is possible to sense liquid fed with the gas mixture in the direction of the storage tank at the flow sensor, as additionally induced cooling capacity. This liquid can be condensate or fuel coming from the tank to be filled which indicates a malfunction of the filling system.
In a preferred version of the method, an error signal is transmitted in the case of a value for the throughflow of the gas mixture which deviates by more than a prespecified error value from a target value predetermined by the delivery rate of the fuel pump. Greater deviations indicate operational disturbances. Furthermore, connection errors during the assembly or in the configuration of the filling system, for example, can be recognised in this way. If e.g. the delivery rate of the fuel pump is coded by a false pulse valency (pulse frequency), the erroneous target value predetermined by the erroneously coded delivery rate of the fuel pump deviates substantially from the actual value determined in the measurement of the throughflow of the gas mixture, even in the case of volumetrically matching delivery rates of fuel pump and gas pump.